IPD (also known as SAR, Systemic Acquired Resistance) in a crop results from altered metabolism of plant tissue and is manifested by various defense mechanisms, including the accumulation in the crop of soluble proteins referred to as pathogenesis-related (PR) proteins. Some PR proteins have been shown to be hydrolytic enzymes, such as chitinases and .beta.-1,3-glucanases, while others are shown to peroxidases. Also accumulated are a group of these proteins having a molecular weight of about 10 to 20 kDa, referred to as P14 proteins, which are now known to be anti-fungal. All or part of these proteins are believed to participate in the defense systems of a crop. Various isonicotinoyl-pyridinyl-hydrazine-derivatives, such as 2,6, -dichloroisonicotinic acid (INA), and benzothiadiazole compounds, such as Ciba-Geigy (CGA 245704), have been described in the patent literature as immunizing healthy plants against fungal diseases (European Patent Publication Numbers 268 775, 0 288 976; and 313 512). The use of threo-DL-.beta.-methylaspartic acid and of DL-.beta.-aminobutyric acid for the control of root rot of peas caused by Aphanomyces euteiches Drechs has also been described (Papavizas, Plant Disease Reporter 48:537-541, 1964; Papavizas, Plant Disease Reporter 51:125-129, 1967).
The use of D-alanine, D- and DL-leucine and DL-.alpha.-aminoisobutyric acid at 0.03 M was described to reduce scab in apple caused by Venturia inaequalis (Kuc et al., Phytopathology 49:313-315, 1959).
Van Andel showed (Tijdschur. Plantenziekten 64:307-327, 1958) that DL-serine, D-serine (and to a lesser extent L-serine), phenylserine, DL-threonine, but not DL-.alpha.-aminobutyric acid nor DL-.beta. aminobutyric acid, behaved as chemotherapeutants against the fungus Cladosporium cucumerinum on cucumber (ibid. page 318).
Oort and van Andel (Mededel Landbourhoogeschool Opzockkinssta, Staat Gent. 25:981-992, 1960) showed that DL-.beta.-aminobutyric acid applied to leaves of tomato protected those leaves against Phytophthora infestans (ibid., page 987).
Various derivatives of DL-.beta.-aminobutyric acid and .beta.-aminocrotonic acid have been described in the patent literature as fungicides against P. infestans in tomato and Plasmopara viticola in grape (German Patent No. 1,120,802).
Recently, Cohen et al (Plant Physiol., 104:58-66, 1994) reported that PR proteins are involved in IPD in tomato.
Systemic Acquired Resistance is very often specific to a crop and a disease. For example, INA and CGA 245704 can immunize tobacco against the fungus Peronospora tabacina but not potato or tomato against P. infestans (Y. Cohen, unpublished). Also known in the literature is that various isomers of a compound may show different abilities to induce systemic resistance. For example, DL-.beta.-aminobutyric acid induces resistance against Fusarium wilt in tomato, whereas .alpha.-aminoisobutyric acid induces resistance against powdery mildew in wheat (Kalix et al, in Modern Fungicides and Antifungal Compounds, Lyr et al eds, Intercept, 1995). The literature also teaches that even enantiomers of the same molecule greatly differ in their IPD ability. Thus, Cohen showed (Physiol. Molec. Plant Pathol., 44:273-288, 1994) that (R)-.beta.-aminobutyric acid can immunize tobacco against P. tabacina, whereas (S-)-.beta.-aminobutyric acid cannot. Oort and Van Andel concluded: "We do not understand why one of two related amino acids has an evident effect and the other has not, and why one influences a given plant pathogen combination but has no or hardly any influence on another" (op. cit., page 987).
The method of the present invention is not obvious in view of the prior art cited for the following reasons:
1. Oort and van Andel showed that BABA (DL-.beta.-aminobutyric acid) applied exclusively to the leaves of tomato two days before inoculation reduced infection with P. infestans (ibid., page 987, line 5). The present invention shows that BABA protects plants against disease when applied to either the leaves or to the roots, or even injected to the stem, and may be applied either before or after inoculation. Also the data of Oort and van Andel are merely qualitative, as no BABA concentration or percent protection are given.
2. U.S. Pat. No. 3,899,585 (Misato et al; Aug. 12, 1975) teaches in Table 1 that Test Compound No. 13 (which is the closest to the present invention), 2-aminobutyric acid lauryl ester hydrochloride, applied to rice leaves reduces rice blast disease. The present invention teaches that 2-aminobutyric acid was totally ineffective in reducing diseases in various crops (see Cohen, Phytopathology 83:55-59, 1994; Cohen, Physiol. Molec. Plant Pathol. 44:273-288, 1994). It also teaches that Test Compound No. 13 should be applied preventively, rather than curatively, unlike the present invention which allows application of the test compound curatively. Interestingly enough, Test Compound No. 13 is probably inactive in cucumber against downy mildew, as it is missing from Example 4 (Table 4), unlike the present invention showing that BABA is active against this disease.
3. U.S. Pat. No. 3,991,208 (Dudzinski et al, Nov. 9, 1976) teaches that a tertiary amine group attached to a 2-carbon of a long-chain alkyl and carboxyethyl group substituent of the nitrogen atom are surface compounds which are antibacterial (against Gram positive bacteria). Such compounds are not amino acids, have no effect on fungi, and do not control fungal plant diseases either directly or indirectly and, therefore, have no relevance to the present invention.
4. U.S. Pat. No. 4,481,291 (Watkinson; Nov. 6, 1984) teaches that timber decay due to fungal attack by mainly Basidiomycetes may be prevented by a composition containing a nitrogenous compound and a saccharide compound. The nitrogenous compound selected from a group consisting of DL-methionine sulfoxide, 5-hydroxy lysine HCl, and aminoisobutyric acid. The latter compound is (CH.sub.3).sub.2 --C(NH.sub.2)--COOH (Col. 2, line 28), which is 2-amino-isobutyric acid. According to the present invention, this compound is not suitable for inducing systematic resistance against fungal plant pathogens, but BABA does, not to say that the present invention deals with protection of live green plants and not of dead woody tissue like timber. Moreover, according to Watkinson, the composition must contain a sugar as an energy source for the fungus, whereas the present invention does not.
5. U.S. Pat. No. 5,097,700 (Seibel et al; Mar. 17, 1992) teaches that halogenated amino acid derivatives are useful antibacterial agents in humans. This prior art is, indeed, not relevant to the present invention because, first, it deals with bacterial human disease and not with fungal diseases of crop plants; second, it involves halogenated (at least one halogen atom bound to the carbon backbone at position 2) amino acids, which is not required for BABA to induce IPD.
6. GB 1,048,507 (Harinack et al.; Nov. 16, 1965) teaches that glycine derivatives are effective systemic fungicides in crop plants although they are not effective against fungus spore germination in in vitro tests (page 1, line 40). This prior art departs from the present invention due to the fact that glycine is a 2-amino acid (.alpha.-amino acid) in which the NH.sub.2 group is bonded to carbon 2, while the present invention deals with 3-amino acids (.beta.-amino acids) in which the NH.sub.2 group is bound to carbon 3. According to the present invention, only 3-aminobutyric acids, but not 2-aminobutyric acids, have systemic IPD effect.
The present invention deals with compounds that protect crop plants against fungal attack via immunization, namely altering plant metabolism so that it can resist fungal colonization in its tissues. It was shown by Cohen et al (Plant Physiology 104:59-66, 1994) that BABA enhances the accumulation of PR proteins in tomato. This accumulation was correlated with resistance to P. infestans. However, this is probably not the case in other crops, such as curcurbits and tobacco, in which BABA also induces IPD response. In curcurbits BABA induces the accumulation of callose and lignin in the infected sites, which probably stops the fungus, whereas in tobacco, the IPD mechanism remains obscure (Cohen, Physiol. Molecul. Plant Pathol. 44:273-288, 1994).